chore: pin to Python 3.12 for docs commands only; fix contributing do… #4707

Job	Run time
mypy	1m 7s
slotscheck	25s
pyright	44s
Test server integration (uvicorn<0.27.0)	43s
Test Minimal Application with Base Dependencies	29s
Test server integration (uvicorn>=0.27.0)	1m 58s
Test Minimal Pydantic 1 application	1m 18s
validate	19s
test (3.10) / test	2m 11s
test (3.11) / test	1m 39s
test (3.13) / test	2m 17s
test (3.12) / test	3m 34s
test (3.8) / test	3m 16s
test (3.9) / test	2m 1s
test-platform-compat (macos-latest) / test	1m 5s
test-platform-compat (windows-latest) / test	5m 56s
build-docs	0s
sonar	1m 57s
upload-test-coverage	17s
codeql	2m 18s
	33m 34s

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